The need for new concepts in the construction of, and new materials for use in, the packaging of integrated circuits for high performance applications has been continuously driven by the ever-increasing demand for faster speed of operation and higher circuit density. For example, technology has been developed to pack ever greater numbers of transistors, the basic active electronic unit, onto a single silicon or gallium arsenide wafer or chip. Performance of the resulting devices (integrated circuits) requires them to be electrically connected to other devices and eventually to an operating medium. Integrated circuits must be protected from overheating, from physical abuse, and from the environment. The necessary connections and protections are supplied by encasing the unit in a package.
One component of such a package is a substrate or "chip carrier" which consists of a thin plate or wafer of an electrical insulator upon which one or more chips are mounted, and which serves as a base for metallized transmission lines which electrically interconnect the chips to one another and to the other components of the electronic device.
The largest volumes of integrated circuits produced today utilize packages prepared from organic plastics. Nevertheless, for long life, high reliability applications, hermetically sealed packages fashioned from inorganic ceramic materials have been employed. Such packages most frequently have been produced from alumina (Al.sub.2 O.sub.3).
The speed at which the electronic device operates is in large part limited by the speed at which a signal can be propagated along those interconnects and the distance the signal must travel. Two means by which the signal speed can be increased comprise: (1) decreasing the electrical resistivity of the interconnect; and (2) utilizing a substrate material with a very low dielectric constant, inasmuch as the rate at which a signal propagates through the interconnect is inversely proportional to the square root of the dielectric constant of the surrounding medium.
Circuit density can be increased through the following three means: (1) employing a greater number of circuit components per unit area of chip; (2) enlarging the dimensions of the chip to thereby increase the number of chip components; and (3) utilizing a greater number of chips per substrate, which action also serves to accelerate signal speed since the chips are thereby placed in closer proximity to one another than where they are mounted on separate chip carriers. Increasing the circuit density, however, places two additional heavy demands on the package.
First, higher circuit density requires a greater number of interconnects, and avoiding crossover of the interconnects (with consequent short circuiting) often necessitates the use of a multilayer substrate. Such a laminated substrate construction involves metallized pathways passing over one another along different planes within the interior of the substrate. This construction requires co-firing of the metallized interconnects with the substrate material. That requirement limits the selection of metals suitable for interconnects to those having melting points higher than the temperature at which the substrate is sintered. At the present time the only ceramic material utilized in multilayer substrates for high performance applications is Al.sub.2 O.sub.3. Because Al.sub.2 O.sub.3 requires a sintering temperature of 1600.degree. C. or higher, materials suitable for interconnects are limited to refractory metals such as tungsten and molybdenum. Those metals exhibit higher electrical resistivities than do copper, silver, or gold. Accordingly, signal speeds are not as fast as could be achieved if those latter, less refractory metals could be used.
Second, increased circuit density requires more effective heat removal from the package. Hence, higher switching speeds on the chip, coupled with a larger number of components and chips, result in greater heat generation. Excessive heat buildup can lead to chip failure or fracture of the solder bond between the chip and the substrate due to differences in thermal expansion. Consequently, heat dissipation is a major concern in numerous high performance applications and, quite apparently, could be improved by preparing the substrate from a material exhibiting a substantially higher thermal conductivity than that of Al.sub.2 O.sub.3.
From the above discussion it is believed evident that significant advances in integrated circuit packaging could be achieved by fabricating substrates from materials which, when compared to Al.sub.2 O.sub.3, demonstrate the following characteristics:
(1) a lower sintering temperature, specifically below 1050.degree. C., so as to permit the use of metals of lower electrical resistivity for interconnects;
(2) a lower dielectric constant, preferably below 6; and
(3) a substantially higher thermal conductivity.
The currently-available materials exhibiting low dielectric constants and low firing temperatures are principally found among glasses and glass-ceramics having compositions predominantly, but not exclusively, in the silicate system. Unfortunately, however, those materials inherently demonstrate very low thermal conductivities, i.e., at least one order of magnitude less than that of Al.sub.2 O.sub.3. Consequently, the use of those materials is restricted to applications where the speed at which the electronic device operates is important, but effective heat dissipation is either not a critical factor or can be achieved by the incorporation of heat sinks and external cooling mechanisms. Whereas these devices may provide effective heat removal, they are expensive and cannot be used in many applications.
Conversely, materials exhibiting high thermal conductivities are available in the form of metals and also in certain ceramics which are characterized by covalent bonding, simple stoichiometries, and low average atomic mass. The metals, however, are electrically conductive and, hence, are not suitable as package substrates. Furthermore, although most of the ceramics demonstrating high thermal conductivity are electrically insulating, few have a dielectric constant which is significantly lower than that of Al.sub.2 O.sub.3, and all demand sintering temperatures well in excess of 1050.degree. C. This latter characteristic necessitates the use of more refractory and, concomitantly, more electrically resistive metals as interconnects in co-fired substrates. This situation results in highly conductive ceramic packages being restricted to applications where heat removal is more vital than enhanced operating speed of the electronic device.